Arc discharge lamps operate by jumping an electric arc between two displaced electrodes. The electric arc excites the atoms of a gas along the arc path to high energy states resulting in light and heat. Arcing ideally occurs directly between the electrode tips, which is generally accomplished by making the gap between the electrode tips the smallest distance between the electrodes.
Due to heating of the electrode shaft, the gas surrounding the shaft, or to surface conductivity, arcing may occur away from the ideal tip of the electrode. The electrode material adjacent the arc generation point may become hot and electron emissive. The adjacent area may then act as an additional source for the arc generation and become hotter. In time the new source point may become the sole arc source point. The process may continue with the arc wandering along the length of the electrode, even when the distance between the two arc points is no longer minimal. The arc may then moved from the optically ideal position, thereby upsetting the optical design. In the worst case, the wandering arc may settle near the envelope material. The envelope may then be heated beyond the material tolerance, resulting in lamp failure.
A double ended arc discharge lamp design uses collinear, but separated electrodes. The input leads are then substantially separated to strongly control the arc position, but making convenient bases difficult to design. Alternatively, the electrodes may be positioned generally side by side for a single ended design thereby allowing a more conventional base, and potential arc wandering.
In most single ended, side by side electrode designs, the active area of the electrode is the tip of a shaft where emission and collection of electrons occurs. It is known in the art to bend or angle the shafts towards one another thereby making the tip to tip distance the smallest separation between the electrodes. Several forms of side by side electrodes exist, and example electrodes may be seen in U.S. Pat. Nos. 2,687,489; 2,876,427; 3,170,081; 3,405,303; 3,849,690; and 4,275,329.
The electrode shafts may be parallel, ending with blunt or formed electrode tips. Unfortunately, the arc may wander along the length of the parallel electrode shafts. The electrode shafts may angle or curve towards one another ending with the electrode tips pointing one to the other. Angling or curving the shafts requires sufficient envelope volume to contain the required tip separation and the angled shafts. Arc wandering may still occur along the portions of the shaft adjacent and continuous with the electrode tip. The electrode shafts may be parallel and then bend at right angles toward each other so the electrode tips point one to the other, forming an L shaped electrode. The whole foot of an L shaped electrode may become emissive, with arc wandering still occurring along the parallel shaft sections. There is then a need to provide an electrode form useful in arc discharge lamps with side by side electrodes that prevents arc wandering.